feat: CI job for building Docker binary-only images

[Wirone]

Similar to composer/docker#250 (more info), this PR introduces a CI process for releasing binary-only images for Pie 🥳!

You can see the result here, there is an image in my fork's Package Registry, built here.

Here's how the image looks like (dive ghcr.io/wirone/php-pie:latest-bin):

PS: I decided to not verify GPG signature during build because we can trust our own workflow (we copy PHAR file built in previous stage), but also I had some problems with verification (GPG imported correctly, but verification failed). Users may verify the file on their own, if they want, using attached pie.asc. Signing was dropped.

Fixes #137

[Wirone]

Rebased interactively with squashing into easier to grasp commits (workflow + usage), some changes enforced by changes in the upstream were applied during the rebase. The effect of current state of the PR can be seen here and here. At this point I didn't address attestation.

[Wirone]

@asgrim Docker build attestation from latest build can be found here.

[Wirone]

@asgrim Is there any actionable thing on my side? I believe this can wait for non-labs release, as this is only a cosmetic change.

[TimWolla]

I'm still concerned about https://github.com/php/pie/pull/122/files#r1854054535, given that we only make the Phar available as “latest” and as a specific version, not by any “major” / “minor” wildcards.

The workflow steps look fine to me, except for the few remarks I've made.

[Wirone]

@TimWolla @asgrim tested new release, images have been built correctly, you can see them in the registry. dive inspection:

[TimWolla]

LGTM now. Thanks for your patience with my reviews 😄

[Wirone]

@TimWolla after @keradus' reviews nothing seems to be annoying 😂. But seriously, thanks for the great feedback, I've learned new things during the process so for me this was added value. I like good code reviews 👍.

[asgrim]

Thank you @Wirone for your patience, this LGTM now 👍 got there in the end 😁 nice work, and thanks @TimWolla for your great feedback too!